1. Field of the Invention
The present invention relates to a process for mounting an ultra-high frequency diode so as to form a pre-matched module, by using the dimensions of the case in which the diode is sealed. The type of mounting according to the invention applies to avalanche, Gunn, Schottky and other diodes such as varactors and varicaps. The object of the invention is to seal a diode in a case so that a pre-matched module is formed, that is to say a module allowing the low impedance of the diode to be matched to the high impedance of the surrounding medium, generally air. The sealing case is formed with a thin dielectric ring, quartz being particularly well suited since it gives a low parasite capacity and a connection of the false beam-lead type so as to minimize the parasite inductance. The shape of the beam-lead also reduces the parasite capacity.
2. Discussion of Background
Impedance matching is necessary with ultra-high frequencies, for example in the 94 GHz band, for a diode which is to radiate in the space surrounding it. This matching is particularly advantageous in radial form for it may be obtained by means of an ad hoc case. At the level of the semi-conductor chip, a diode in the present instance, the impedance which it presents is of the R+jX type in which R represents the real parts of the resistance of the diode and X the imaginary part of this resistance, j being equal by definition to j.sup.2 =-1 and is representative of the imaginary quantities. R is negative for an avalanche diode or a Gunn diode and positive for a Schottky diode or a diode of another type. FIG. 1 will assist in better understanding the importance of impedance matching for sealing a diode in a case.
In this FIG. 1, let A be the diode in which the semiconductor chip is soldered to a metal base and comprises an upper connection. Let .phi..sub.i be the diameter of the metal stud to which diode A is soldered, .phi..sub.e the diameter of the pedestal that is to say also of the inner diameter of the cavity, let B be the distance which separates the upper connection from the base and C the thickness of the diode, or else the distance which separates the upper connection from the stud to which the chip is soldered. The diode chip is surrounded by a dielectric of given constant, or several dielectrics of different constants, which give a mean equivalent constant. Depending on the case, the dielectric which surrounds the chip is either air or an insulation in the form of resin. It can be demonstrated that, with the impedance of the space surrounding the diode having a value R.sub.e +jX.sub.e referred to the periphery, there exists a single set of values of .andgate..sub.i, .phi..sub.e, B, C such that there is impedance matching of the semi-conductor device, namely: EQU R.sub.e =k.sub.1 R and X.sub.e =k.sub.2 X
However, in practice, it is enough if k.sub.1 R and k.sub.2 X are sufficiently close to R.sub.e and X.sub.e for then the conventional mechanical tuning in the mount becomes extremely easy; this is why we say pre-matching and not matching.
This impedance transformation is extremely advantageous in particular for avalanche diodes, for it considerably simplifies the cavity: for example, biasing on the upper electrode may be provided by a simple fine wire. Furthermore, and this is shown in FIG. 2, the curve of the power delivered as a function of another variable such as the biasing current I or the ambient temperature T, is very clean without hysteresis or without snags. Thus, in FIG. 2 curve 1 represents the power curve of diodes sealed in a case without impedance matching: this curve does not have a regular variation but on the contrary presents deviations which vary moreover from one diode to another depending on the case. On the contrary, curve 2 which has a very regular variation corresponds to a diode correctly sealed in a case with impedance matching.
This shows the importance, especially at such frequencies close to 100 GHz, of a diode chip being correctly mounted and sealed in its case, which forms a cavity, preferably from parts formed collectively so as to be more industrial.